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// Copyright 2022 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#include "base/memory/platform_shared_memory_mapper.h"
#include <sys/mman.h>
#include "base/compiler_specific.h"
#include "base/logging.h"
#include "base/numerics/safe_conversions.h"
namespace base {
std::optional<span<uint8_t>> PlatformSharedMemoryMapper::Map(
subtle::PlatformSharedMemoryHandle handle,
bool write_allowed,
uint64_t offset,
size_t size) {
// IMPORTANT: Even if the mapping is readonly and the mapped data is not
// changing, the region should be mapped with MAP_SHARED.
//
// With ashmem doing a mmap(MAP_PRIVATE) would be equivalent anonymous
// mapping, not a shared one.
//
// Historically, memfd had the opposite and confusing behavior: attempting to
// mmap(MAP_SHARED) a sealed, read-only region would fail, while
// mmap(MAP_PRIVATE) would correctly create a read-only shared mapping.
//
// This inconsistency was resolved in Linux kernel 6.7, which now allows
// MAP_SHARED to work as well. This fix may also be cherry-picked to
// older LTS kernels.
//
// See the upstream patches:
// https://lkml.kernel.org/r/cover.1697116581.git.lstoakes@gmail.com
//
// For old devices PlatformSharedMemoryMapper only receives ashmem regions.
// New devices shipping with Android 17 do not have ashmem, and memfd should
// be used instead. These new systems are based on 6.12+ kernels (in other
// words, MAP_SHARED works for them). Devices updating to Android 17
// (potentially with kernels as old as 6.6) continue using ashmem.
//
// Therefore, it is unnecessary to dynamically check the memory region type
// (ashmem vs. memfd), as MAP_SHARED is the correct flag for all scenarios
// this code handles.
void* address =
mmap(nullptr, size, PROT_READ | (write_allowed ? PROT_WRITE : 0),
MAP_SHARED, handle, checked_cast<off_t>(offset));
if (address == MAP_FAILED) {
DPLOG(ERROR) << "mmap " << handle << " failed";
return std::nullopt;
}
return UNSAFE_TODO(span(static_cast<uint8_t*>(address), size));
}
void PlatformSharedMemoryMapper::Unmap(span<uint8_t> mapping) {
if (munmap(mapping.data(), mapping.size()) < 0) {
DPLOG(ERROR) << "munmap";
}
}
} // namespace base
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